1. Field of the Invention
The present invention relates to radiation curable liquids and inkjet inks and their use in inkjet printing methods for food packaging applications.
2. Description of the Related Art
In inkjet printing, tiny drops of ink fluid are projected directly onto an ink-receiver surface without physical contact between the printing device and the ink-receiver. The printing device stores printing data electronically and controls a mechanism for ejecting the drops image-wise. Printing is accomplished by moving a print head across the ink-receiver or vice versa or both.
The use of radiation curable inkjet inks is preferred for inkjet printing on non-absorbing ink-receivers. In industrial ink jet systems, there is a constant demand for increased printing speeds in combination with high image quality. The new print heads designed for increasing printing speed only operate with very low viscous inkjet inks. Suitable monomers to obtain such very low viscous ink jet inks have been described in EP 0997508 A (AGFA) that discloses radiation curable monomers containing vinylether and acrylate functions.
However, migrateable residues in cured layers of inkjet ink on packaging of foodstuffs may present a health risk and consequently they should be kept to an absolute minimum, i.e. within limits of applicable legislations such as the Swiss ordinance SR 817.023.21 on Objects and Materials. UV-curable inks generally contain colorants, monomers, photoinitiators and polymerization synergists. A known measure to reduce extractables of the photoinitiating system from cured ink layers is the use of diffusion hindered compounds, such as polymeric or polymerizable photoinitiators and co-initiators, instead of the usual low molecular weight compounds. For example, US 2006014848 (AGFA) discloses radiation curable inkjet inks comprising a polymeric co-initiator comprising a dendritic polymer core with at least one co-initiating functional group as an end group. Aliphatic amines and aromatic amines are included as suitable co-initiating functional groups. The dendritic polymeric architecture allows to obtain low extractables and at the same time to minimize the increase in viscosity of the ink.
The colorants used in curable inkjet inks can be dyes, but are generally colour pigments which together with a polymeric dispersant attached to the surface of the pigment are usually very difficult to extract. The remaining problem for extractables is the monomers. The use of polymerizable oligomers or crosslinkable polymers instead of low molecular weight monomers is only possible up to a certain amount in the ink due to limitations of inkjet printing requiring the inks to possess a low viscosity at the jetting temperature.
Specific mixtures of monomers as in EP 2053101 A (AGFA) and EP 2053103 A (AGFA) were found for minimizing the amount of unreacted monomers that can be extracted. Extractable monomers can however cause problems in two different ways: set-off and migration. Set-off occurs in roll-to-roll printing where the printed front-side of a packaging material comes into contact with the unprinted back-side and unreacted monomers are set off on the backside intended for direct food contact. The unwanted transfer of unreacted monomers to the food is by migration through the packaging material.
Popular packaging materials suffering from such migration are usually olefin based substrates like polyethylene or polypropylene film. Due to the low viscosity of radiation curable inkjet inks, monomers easily penetrate into the substrate before they can be effectively cured.
One approach to reduce monomers migrating into a packaging material is by replacing them with water. U.S. Pat. No. 6,803,112 (SUN CHEMICAL) discloses a method for producing a low-extractable film packaging from an actinic radiation curable aqueous composition containing a water soluble compound having at least one α,β-ethylenically unsaturated, radiation polymerizable group and water as essential components carried out by applying the aqueous composition to a surface which is then irradiated in a single step with actinic radiation in the presence of the water thereby forming a cured film wherein less than 50 ppb of the water soluble compound or its residual components are extractable by a food simulant. However, the inclusion of large amounts of water in the curable inkjet ink leads to latency problems in the print head and to inferior image quality due to the spreading characteristics of water on substantially non-absorbing ink-receivers.
There is widespread belief that cationic inkjet inks would be more suitable for food packaging applications. Cationic inkjet inks tend to polymerize slower than free radical polymerizable inkjet inks but to a larger extent. However, there is currently no evidence that cationically polymerizable monomers would pose no or less problems for migration into packaging materials.
US 2003199655 (NIPPON CATALYTIC CHEM) discloses a reactive diluent composition comprising a vinyl ether group-containing (meth)acrylic ester and a hydroxyl group-containing polymerizable compound and/or divinyl ether, for use in an activated energy ray-curable ink composition for ink-jet printing.
Therefore, it would be desirable to reduce or eliminate the set-off and migration of unreacted monomers of radiation curable inkjet inks in printing on polyethylene and polypropylene based food packaging materials so that health risks are minimized.